This invention relates to a method of preparing modacrylic fibers, that is fibers which are composed of polymers containing 50 to 85% by weight of acrylonitrile units in their molecule, exhibiting a high lustre, high stability towards thermal treatments, and high resistance to flame.
Known are several methods for preparing fibers with essentially the above features, in particular the processes described in copending Italian Patent Applications Nos. 28980 A/76 and 27997 A/77 by this same Applicant. Such processes comprise the steps of admixing in suitable ratios a binary copolymer resulting from the copolymerization of acrylonitrile (ACN) and vinylidene chloride (VDC) with a polymer obtained from copolymerization of acrylonitrile and a suitable sulphonic derivative, perferably a derivative of the alkyl-acrylamidoalkane-sulphonic series. With these methods, a fiber is obtained which is characterized by a limiting oxygen index (L.O.I.) equal to at least 26% or more of oxygen.
This invention is preferably applied in conjunction with a method, such as the one described in the cited copending Applications; however, the invention is not restricted to that application, as it may also be applied, as a rule, to different processes and the resulting different modacrylic ploymers.
It is known that the flame resistance of a halogenated modacrylic fiber can be improved through the addition of antimony oxides. However, the addition of a trivalent antimony oxide to the fiber in the amounts required to achieve flame-resisting properties produces a highly opaque or matt fiber.
To solve the problem of the obtainment of a flameproof modacrylic fiber, of the so-called "L.O.I. uprated" type, but having unaltered lustre characteristics, several solutions have been proposed in the art. For example, it has been proposed that organic derivatives of antimony be prepared which are soluble in commonly used spinning solvents, but this approach involves difficulties of preparation and high costs. Also proposed has been the utilization of pentavalent antimony oxides (Sb.sub.2 O.sub.5) in particle sizes smaller than 100 millimicrons, i.e. smaller than the wavelength of light, therefore they cannot function as diffraction nuclei and consequently do not modify the lustre characteristics of the resulting fiber.
Moreover, the addition of antimony pentoxide to the spinning solution or "dope" poses considerable technical difficulties. In practice, the only known way of using it is that of suspending it in water or a specially provided organic carrier and adding the resulting suspensions to the spinning dope. However, this approach introduces considerable problems, such as coagulation phenomena, solution instability, etc.